The invention relates to semiconductor laser transmitters for use in fiber-optic communication systems.
Directly modulated semiconductor lasers are attractive for use as transmitters in optical communication because they are compact, have large response to modulation, and are integrable. In addition they are inexpensive compared to externally-modulated transmitters, which require an intensity modulator, usually LiNbO.sub.3, following the laser. However, they suffer from the major drawback that their outputs are highly chirped. Chirp is the rapid change in optical frequency or phase that accompanies an intensity-modulated signal. Chirped pulses become distorted after propagation through tens of km of dispersive optical fiber, increasing system power penalties to unacceptable levels. This has limited the use of directly modulated laser transmitters to propagation distances of only tens of km at 2.5 Gb/s as described by P. J. Corvini and T. L. Koch, Jounral of Lightwave Technology vol. LT-5, no. 11, 1591(1987). The distortion-less transmission distances become even shorter for higher bit rates.
An alternative transmission system that produces reduced chirp was described by R. E. Epworth in UK patent GB2107147A in which the modulated laser is followed by an optical frequency discriminator. The laser bias current is modulated by the electrical signal to produce small amplitude modulation as well as modulation of the laser frequency. The discriminator then converts the FM modulation to AM modulation. Epworth cites classic optical discriminators, namely Mach-Zehnder interferometer, Michelson interferometer, and two Fabry-Perot resonators for implementation of this invention. N. Henmi describes a very similar system in U.S. Pat. No. 4,805,235, also using a free-space interferometer.
The above are all free-space discriminators comprising mirrors and partial reflectors which are bulky and require mechanical feed-back control for their stabilization. Also coupling of light from fiber to these devices and back introduces loss as well as additional optical components. Furthermore tuning the discriminator for optimum position requires a mechanical adjustment of the phase differential -in one arm of the interferometer. As stated in UK patent GB2107147A, the modulated laser frequency varies by only 10-20 GHz, so to obtain a 10 GHz frequency discrimination, a.about.1.5 cm free-space delay is needed, which requires use of mechanically driven parts. Piezoelectric elements can only make small motions and are known to drift, requiring further stabilization circuits.
N. Henmi in U.S. Pat. No. 4,805,235, also sites the use of a diffraction grating discriminator. To obtain 10-20GHz frequency discrimination as stated above, the diffraction grating has to have a resolution of a few GHz to discriminate between the "1s" and "0s" in a digital system. From "The Feynman Lectures on Physics," vol. I, Addision Wesley, Mass. (1963), a frequency resolution .DELTA..nu..about.1 GHz requires a time difference between extreme paths of .DELTA.t.about.1/ .DELTA..nu.1ns corresponding to a larger than 1 ft wide diffraction grating. As described in U.S. Pat. No. 4,805,235, light beams of different frequency components have to diffract before they are separated by slits a distance away from the diffraction grating. This makes for a bulky device. Also, as for the other freespace space optics discriminators mentioned above, it suffers from fiber coupling loss and requires mechanical tuning.
In U.S. Pat. No. 5,317,384, J. P King describes a fiber Mach-Zehnder interferometer as discriminator comprising polarization-preserving fibers, couplers, and a fiber delay line. This discriminator is an improvement over the previously mentioned discriminators in that it is in-fiber and is polarization insensitive. Discrimination is achieved by making one arm of the interferometer longer length of fiber (we calculate, by .about.1 cm longer for 10 GHz variation). This has the disadvantage that the discriminator cannot be tuned. Also, it is a complicated structure comprising two fiber polarization splitters, two fiber couplers, four cross splices and a regular splice.
Furthermore the transfer function of a Mach-Zehnder discriminators is limited to being sinusoidal. For digital applications, a sinusoidal transfer function is not optimum and leads to distortion if the frequency excursion of the laser exceeds the range of the transfer function between a first maximum and minimum.